4.3 Noise shield measurements
4.3.3 Results for tonal noise excitation
The measurement results for the noise reduction difference of the noise shields with respect to the hNRi of the fuselage under tonal noise ex-citation at f = 100 Hz with different trace wave numbers are shown in Fig. 4.15. The first row in Fig. 4.15 shows the results for the exci-tation sound wave propagating purely in axial direction (kx) at three different trace wave numbers 2.5, 5.5, and 8.5 rad/m. In the second row, the excitation sound wave propagates in a 45◦ angle along the noise shield (kxy) at the same three trace wave numbers. The third row shows the results for excitation waves propagating in circumferential direction (ky).
In the pink noise excitation results in Fig. 4.13(b), it could be seen that at 100 Hz the noise reduction difference of the MAM noise shield is approximately 3 dB. The noise reduction of the other noise shield
−30 3 6 9
kx
−30 3 6 9
kxy
−30 3 6 9
2.5 rad/m 5.5 rad/m 8.5 rad/m ky Double wall
Shield w/o MAMs Shield w/ MAMs
∆hNRiindB(ref:fuselage)
Figure 4.15: Measured average noise reduction differences for the different noise shield configurations at tonal noise excitation (f = 100 Hz) with differ-ent trace wave numbers.
configurations is slightly below 1 dB. The tonal excitation results with different wave numbers in Fig. 4.15 indicate that the noise reduction differences of all three noise shield designs strongly depend on the char-acteristics of the excitation field. For example, in the case of an exci-tation with kxy = 2.5 rad/m or ky = 2.5 rad/m, the ∆hNRi-value of the MAM noise shield is similar to the value obtained under pink noise excitation. On the other hand, in all threekx-cases, the ∆hNRi of the MAM noise shield is only half of that value. For the other two noise shield configurations shown in Fig. 4.15, a negligible noise reduction difference can be observed for the kx-cases. For ky = 2.5 rad/m, how-ever, the double wall yields the biggest noise reduction improvement of all three noise shields, even though it is the lightest design and does not contain any MAMs.
Therefore, no clear conclusions can be drawn from these measure-ment results under tonal noise excitation. For example, in all three kx-cases the measured ∆hNRi-values seem to be independent from the actual trace wave number. This is consistent with the analytical predic-tions in previous chapters, where it was shown that the anti-resonances of MAMs are not affected by the nature of the incident sound field, as long as the unit cells are smaller than the acoustic wavelength. For the other wave directions, kxy and ky, however, the ∆hNRi of the MAM noise shield varies considerably with the magnitude of the trace wave number. In the case of ky-waves, for example, the MAM noise shield has a ∆hNRi of nearly 4 dB at the smallest trace wave number and drops to slightly above 1 dB at the largest value ofky. Since the other two noise shield designs also exhibit large variations for these two wave direction cases, it can be expected that this is more likely a result of the global behavior of the noise shield response to the acoustical exci-tation. It is possible that the noise shield structure exhibits different structural and acoustical modes, depending on what is integrated into the air gap between the fuselage and the cover sheet. Furthermore, it
is possible that the acoustic flanking paths in the measurements also depend on the excitation characteristics. An effect inherent only to the MAM layers, therefore, seems unlikely to be the cause for the strong wave number and direction dependence of the noise reduction perfor-mance of the noise shield.
The aim of this work was to investigate the applicability of MAMs as acoustic treatments inside an aircraft fuselage noise shield for the reduc-tion of low-frequency tonal noise produced by counter-rotating open ro-tor engines. Analytical models for the efficient prediction of the oblique incidence sound transmission properties of MAMs (both unit cell and multi-cell arrangements) and multi-layered structures with integrated MAM layers were developed. These models were verified and validated using FEM simulations and experiments, respectively. Parameter stud-ies were performed using these analytical models in order to identify the most important parameters relevant for the design of noise shields with MAMs. Based upon these results, a MAM noise shield demonstrator with realistic dimensions was designed and evaluated experimentally on an acoustic fuselage demonstrator under different broadband and tonal noise excitation conditions.
In summary, the research questions for this work, as formulated in Section 1.3, can be answered as follows:
• In the analysis of the analytical models of MAM unit cells and multi-celled MAM panels in Chapter 2 it was shown that the anti-resonance characteristics of MAMs do not depend on the characteristics of the sound excitation and the overall dimen-sions of multi-celled panels. This is valid as long as the acoustic wavelength is large compared to the unit cell dimensions and the sound pressure acting on the MAMs is approximately uniform over each unit cell. Thus, under these conditions, MAMs still
ex-hibit significant TL maxima at their anti-resonances, even when more realistic sound excitation fields (e.g. diffuse fields) and finite sized multi-celled panels are considered. Furthermore, it could be shown that the membrane prestress variation in multi-celled ar-rangements, which cannot be avoided entirely due to manufactur-ing uncertainties, does not significantly impair the anti-resonance characteristics of the MAM panel.
• In the long wavelength limit, the effective surface mass den-sity and the transfer matrix approach can be used to accurately and quickly estimate the sound reduction performance of multi-layered structures with MAMs. Certain assumptions inherent to the transfer matrix method, such as laterally unbounded and pla-nar layers, limit the applicability of this method. However, the computational effectiveness of this approach makes it an excellent tool for the preliminary design process of such structures.
• Using the transfer matrix it could be shown that it is indeed pos-sible to considerably enhance the low-frequency transmission loss of a double wall with MAMs. The anti-resonances of the MAM layers remain effective, but the bandwidth of the corresponding TL maxima can be greatly reduced when an improper design is chosen. For example, it was shown that a double wall design with integrated MAM layers should be as asymmetric as possible (e.g.
by placing the MAMs very close to one of the walls) in order to retain a reasonable anti-resonance bandwidth.
• Finally, the measurements of the full-scale noise shield model on an acoustic fuselage demonstrator indicated the effectiveness of MAMs in a much more complex and realistic noise shield setup.
Under pink noise excitation, a noise reduction improvement by up to 3 dB (compared to the noise shield without MAMs) could be
to the anti-resonances of the MAM layers. However, it was pos-sibly diminished by potential acoustic flanking paths in the test setup. Furthermore, in the tonal excitation measurements, a con-siderable dependence of the MAM shield noise reduction perfor-mance on the spatial characteristics of the excitation field could be identified.
All in all, these answers to the research questions of this thesis ac-complish a significant step forward to the industrial application of membrane-type acoustic metamaterials as low-frequency acoustic treat-ments within aircraft noise shields. However, new open questions have been identified in the framework of this thesis, which should be ad-dressed in further research efforts. The most important questions are:
• How significant is the influence of the acoustic flanking paths in the experimental setup on the fuselage demonstrator test bench?
How can these be avoided?
• What is the physical explanation for the large differences in the noise reduction performance of the MAM noise shield in case of the different tonal excitation fields?
• How can the frequency shifting of the MAM anti-resonances in case of non-rigid attachment to the host structure be accounted for in the analytical models?
It can be expected that by answering these open questions, the MAM noise shield concept can achieve technology readiness level (TRL) 5 [56]. This enables the development of industrial-scale prototypes for reaching the next TRL.
In addition to that, the analytical models that were derived in this thesis provide a new and efficient framework to investigate sound insu-lation structures containing MAMs. The models are not restricted to be
applied for noise mitigation in aircraft structures, but they can be used in many different fields of noise control engineering, e.g. noise in cars or trains and building acoustics. Furthermore, the analytical models can be employed to develop innovative concepts for new acoustic metama-terials. For example, the analytical models in this work have been suc-cessfully applied to develop MAMs with adjustable acoustic properties which can modify their (anti-)resonance frequencies in situ by applying a static pressurization to the membrane [38]. This approach might be useful when the tonal components in the noise spectrum change over time, e.g. due to shaft rotation speed variations in CROR engines at the different flight phases. Another technological enhancement of MAMs employs perforations inside the mass and membrane material to in-troduce additional anti-resonances and improve the sound absorption capabilities of MAMs without introducing additional mass penalties [39]. This concept also has been developed based upon the analytical models from this work.
As a final conclusion, the results in the present work have shown that MAMs offer a great potential for improving the low-frequency noise reduction of conventional sound insulation structures. The noise shield demonstrator measurements, however, have also identified that – even almost ten years after their first appearance – the successful application of MAMs in practice still is a challenging problem. But the analytical models that were developed in this work can support the solution of this problem such that one day MAMs could be used to address low-frequency noise issues in aircraft, cars, trains, buildings, and many other environments.
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